Coupled cavity slow wave circuit and tube using same

ABSTRACT

A coupled cavity slow wave circuit, such as a cloverleaf circuit, and a microwave tube using same are disclosed. The slow wave circuit includes an array of cavity resonators arranged successively along the beam path with adjacent ones of the cavities having a common end wall structure. A plurality of generally radially directed coupling slots are cut through the common wall between adjacent resonators to form a plurality of axially aligned arrays of coupling slots angularly displaced around the beam path. Each axial array of slots is angularly displaced about the beam path from the adjacent array by 360/N degrees where N is the number of axially aligned arrays of slots. Each axially aligned array of coupling slots includes a web portion of the common wall for blocking off a line-of-sight path parallel to the beam through at least a portion of each array of coupling slots to inhibit cumulative electromagnetic interaction between undesired beamlets in the arrays of slots and the fields of the slow wave circuit, whereby the efficiency and stability of the tube are increased.

United States Patent [72] Inventor Robert J. Butwell San Jose, Calif. [21] Appl. No. 70,197 [22] Filed Sept. 8, I970 [45] Patented Dec. 14, 197I [73] Assignee Varian Assoclntes Pelo Alto, Calif.

[54] COUPLED CAVITY SLOW WAVE CIRCUIT AND TUBE USING SAME 6 Claims, 12 Drawing Figs.

[52] U.S. Cll 315/15, 333/31 A, SIS/3.6, 330/43 [51] Int. Cl H01] 25/34 [50] Field ofSear-ch 315/35,

[56] References Cited UNITED STATES PATENTS 3,010,047 11/1961 Bates 315/35 3,205,398 9/1965 Allen et al. 315/15 3,324,340 6/1967 Pinel et al. BIS/3.5 3,153,767 10/1964 Kyhl 315/35 Primary Examiner-Herman Karl Saalbach Assistant Examiner-Saxfield Chatmon, Jr. Attorneys-Stanley Z. Cole and Leon F. Herbert ABSTRACT: A coupled cavity slow wave circuit, such as a cloverleaf circuit, and a microwave tube using same are disclosed. The slow wave circuit includes an array of cavity resonators arranged successively along the beam path with adjacent ones of the cavities having a common end wall structure. A plurality of generally radially directed coupling slots are cut through the common wall between adjacent resonators to form a plurality of axially aligned arrays of coupling slots angularly displaced around the beam path. Each axial array of slots is angularly displaced about the beam path from the adjacent array by 360/N degrees where N is the number of axially aligned arrays of slots. Each axially aligned array of coupling slots includes a web portion of the common wall for blocking off a iine-of-sight path parallel to the beam through at least a portion of each array of coupling slots to inhibit cumulative electromagnetic interaction [between undesired beamlets in the arrays of slots and the fields of the slow wave circuit, whereby the efficiency and stability of the tube are increased.

Patented 1U 14, 19 H 2 Sheets-Sheet 1 INVENTOR ROBERT J. BUTWELL ATORNEY Patented Dec. 114, 11.971

2 Sheets-Sheet 2 INVENTOR.

ROBERT J. BUTWELL COUPLED CAVITY SLOW WAVE CIRCUIT AND TUBE USING SAME DESCRIPTION OF THE PRIOR ART Heretofore, cloverleaf slow wave circuits for microwave tubes have included eight arrays of axially aligned coupling slots disposed at 360l8 or 45 intervals about the beam path. The slots provide wave energy communication through an array of cavity resonators sequentially arranged along the beam path to form a slow wave circuit. Such a slow wave circuit is disclosed and claimed in US. Pat. No. 3,233,139 issued Feb. 1, I966 and assigned to the same assignee as the present invention.

One problem with this prior art slow wave circuit is that at relatively high power levels, as of l megawatt or above, and with relatively long pulse widths, as of pulse widths greater than microseconds, it is found that the electric fields within the successive cavity resonators are such as to accelerate electrons down the length of the slow wave circuit along paths generally parallel to the beam path and through the axially aligned coupling slots. As a result, eight beamlets are formed surrounding the main beam. With these relatively high power levels, the electrons within the beamlets are accelerated to relatively high energies and they bombard the ends of the circuit to produce melting and arcing within the tube. At all power levels, the energy extracted from the fields of the circuit to accelerate the undesired beamlets substantially reduces the efficiency of the tube.

Heretofore, it has been proposed, as disclosed and claimed in copending application Ser. No. 70,173 filed Sept. 8, 1970 and assigned to the same assignee as the present invention, to somehow block the line-of-sight path through each of the longitudinal arrays of coupling slots surrounding the beam path to block the individual undesired beamlets. In one embodiment, the radial position of the coupling slots is varied throughout the length of the coupled cavity circuit to block a line-of-sight path through each of the arrays of coupling slots. In another embodiment, the angular positioning of the coupling slots in adjacent cavities is rotated by a relatively small angle as of 3 such that each of the arrays of coupling slots transcribes a spiral path rather than a line-of-sight path parallel to the beam through the coupled cavity circuit. Since the electrons are constrained to a straight path parallel to the beam path, due to the magnetic beam focusing field, each of the beamlets is effectively blocked. In another prior embodiment, it has been proposed to insert in each cavity a blocking vane, extending from the nose portion of the cavity for blocking off each of the line-of-sight paths through the axial array of coupling slots to inhibit formation of the beamlets. While the aforementioned arrangements will inhibit formation of the beamlets and thus increase the efficiency and stability of the tube, they have the disadvantage of being relatively complex with regard to manufacture.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved coupled cavity slow wave circuit and microwave tubes using same.

One feature of the present invention is the provision, in a coupled cavity slow wave circuit of a microwave tube having N number of angularly displaced axially aligned arrays of coupling slots being equally spaced around the beam path, of N number of end walls having less than N full-sized coupling slots therein, such N number of end walls being angularly arranged for blocking off a line of sight path parallel to the beam path, through at least a portion of each of said axial arrays of coupling slots, whereby undesired beamlets through the coupling slots are blocked to inhibit cumulative electromagnetic interaction between the beamlets and the fields of the slow wave circuit.

Another feature of the present invention is the same as the preceding feature wherein there are eight arrays of axially aligned coupling slots disposed at 45 intervals about the beam path and wherein at least eight of the cavity end walls have only seven full-sized coupling slots therein with a solid web portion disposed between the first and seventh coupling slot in each end wall, and wherein the web portions are angularly displaced from the other web portions to block off a line-of-sight path parallel to the beam path through each of the eight axial arrays of coupling slots to interrupt the beamlets therein.

Another feature of the present invention is the same as any one or more of the preceding features wherein each of the coupled cavities comprises a cloverleaf cavity having a generally cloverleaf shape with four inwardly directed nose portions disposed at 45 degree intervals about the axis of the beam.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal sectional view, partly in schematic form, of a prior art microwave tube,

FIG. 2 is an enlarged sectional view of that portion of the structure of FIG. ll taken along line 22 in the direction of the arrows,

FIG. 3 is a plot of power output versus time depicting the waveform for an output pulse derived from the output of the tube of FIG. ll,

FIG. 4 is an exploded side elevational view of a portion of the structure of FIG. 1 delineated by line 4-4 and modified to incorporate features of the present invention, and

FIGS. 5A to 5H a series of sectional view of the structure of FIG. 4 taken along section lines A-H, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown a prior art microwave slow wave tube 1 to be modified to incorporate features of the present invention. More particularly, the microwave tube 1 includes an electron gun assembly 2 for forming and projecting a beam of electrons 3 over an elongated beam path to a beam collector structure 4. A coupled cavity cloverleaf slow wave circuit 5 is disposed along the beam path intermediate the gun 2 and collector 4 for electromagnetic interaction with the beam to produce an am plified output signal. A beam focus solenoid 6 is coaxially disposed of the slow wave circuit 5 for producing an axially directed magnetic field in the beam path 3 for focusing the electron beam through the slow wave circuit 5 to the collector 4.

The slow wave circuit 5 includes a plurality of axially aligned cloverleaf coupled cavity resonators 7 successively arranged along the beam path for cumulative electromagnetic interaction with the beam 3. Each cloverleaf cavity 7 includes a pair of axially spaced end walls 8 with an inwardly protruding scalloped sidewall to provide four conductive nose portions 9 inwardly projecting at 45 intervals around the beam path toward the beam axis (see FIG. 2). Adjacent cavity resonators 7 share a common end wall structure 8. The inwardly projecting nose portions 9, in adjacent cavity resonators 7, are angularly displaced about the beam path by 45.

A pair of radially directed elongated inductive coupling slots 11 are disposed on opposite sides of the nose portions 9 to provide negative mutual inductive coupling between adjacent cavity resonators 7. The coupling slots 11 are radially elongated for increasing the inductive coupling between adjacent cavities. The inductive coupling slots 11, in the prior art, were axially aligned along a path parallel to the beam path and the eight arrays of coupling slots were located at 45 intervals about the beam path 3.

Wave energy to be amplified is applied to the upstream cavity 7 via the intermediary of an input waveguide 12 having a wave permeable vacuumtight window 13 sealed thereacross for maintaining a vacuum within the evacuated microwave circuit 5. Midway along the coupled cavity slow wave circuit 5 the circuit includes a circuit sever 14 which comprises a solid centrally apertured conductive disk without coupling slots to prevent wave energy communication between the upstream slow wave circuit portion and a downstream slow wave circuit portion 16. The disk 14 is centrally apertured to permit passage of the electron beam 3 therethrough. Wave attenuative members 19 and were coupled to the downstream cavity 7 of the upstream circuit portion and the upstream cavity of the downstream portion 16 for absorbing wave energy coupled into the cavities on opposite sides of the sever 14.

In the upstream slow wave circuit portion 15, the microwave energy applied to the upstream cavity establishes a wave on the circuit which cumulatively interacts with the beam to produce bunching thereof. The bunched beam passes from the upstream circuit portion 15 into the downstream circuit portion 16 for exciting a growing wave in the downstream circuit portion 16. The wave in the downstream portion 16 cumulatively interacts with the bunched beam to produce a growing wave on the downstream circuit portion 16. Output wave energy is extracted from the downstream end of the downstream circuit portion 16 via an output waveguide 17 which is sealed by a wave-permeable gastight window assembly 18. The RF output energy extracted from the waveguide 17 is fed to a suitable load, not shown.

In the prior art tube 1, when the power output was in excess of 1 megawatt peak and the pulses had a duration in excess of IO microseconds, as shown in FIG. 3, the peak power output suddenly dropped, for the latter half of the pulse width, to about half of the peak power. It was found that this power loss was associated with arcing within the output circuit portion 16. More particularly, it was found that undesired beamlets of electrons were being accelerated through the axially aligned arrays of coupling slots 11 and were bombarding the end walls of the first and last cavity in the downstream circuit portion 16.

When the power output reached a relatively high level, on the order of l megawatt and the pulse length was on the order of 10 microseconds or more, it was found that the energy in the beamlets was sufficient to melt portions of the end walls of the end cavities. When the cavity end wall was melted, trapped gas was released producing an arc in the tube which caused the output power to drop to about half power.

It has been found that the arcing can be prevented and the efficiency of the microwave tube substantially increased by blocking off a line-of-sight path parallel to the beam 3 through each of the arrays of coupling slots, whereby the beamlets do not form to cumulatively interact with the wave on the slow wave circuit 5. A particularly advantageous arrangement of the present invention for blocking the individual beamlets is shown in FIGS. 4 and 5.

Referring now to FIGS. 4 and 5 the angular orientation of the successive cavity resonators 7 about the beam axis 3 is shown along with the relative angular orientation of the slotted common end wall structures 8 therebetween. Each of the slotted end walls 8 includes only seven of the eight inductive coupling slots 11. The seven slots 11 are arranged in axial alignment with the eight arrays of slots, i.e., one every 45 of are about the axis of the beam with the exception that one of the slots is omitted in each of the end walls 8. Thus, between the first and the seventh slot in each end wall 8, there is a nonapertured web portion of the end wall corresponding to the missing slot and indicated by the numeral 22.

The successive cavities 7 are stacked into the longitudinal array with the nose portions 9 in adjacent resonators angularly displaced relative to each other about the axis of the beam 3 by 45. The cavities 7 and end walls 8 are aligned on eight aligning rods passing through aligning apertures 23 provided in the outer margin or lip of the cavity resonators 7 and end walls 8. In this manner precise angular alignment of the successive cavities 7 and end walls 8 is obtained.

In a succession of eight cavity resonators 7, the common end walls 8 are all identical but the angular position of the missing slot portion 22 is displaced relative to the other end walls in the set of eight such that the missing slot 22 takes up each of the eight possible positions according to eight angular positions of the individual axial arrays of coupling slots 11, such as to efiectively block a line-of-sight path through each of the eight axial arrays of coupling slots 11. In a particularly convenient arrangement, the angular position, about the axis of the beam for the missing slot 22 advances by 45 for each of successive resonators 7 taken in the direction of the beam through the longitudinal array of resonator 7. In this manner the missing slot advances in a spiral position throughout the array of resonator 7.

It is desirable that the number of missing slots 22 per resonator be reduced to a minimum, such as one missing slot 22 per end wall 8, in order to maintain the inductive coupling between successive resonators at as high a value as possible. Thus, the preferred embodiment, for an eight-slot coupled cavity array, employs seven slots 11 in each of the common end walls 8, as shown in FIGS. 4 and 5. However, it would be possible in certain applications to have more than one missing slot 22 per common end wall 8, even though this tends to reduce'the inductive coupling between adjacent resonators 7.

The arrangement, shown in FIGS. 4 and 5 for blocking the beamlets through the coupled cavity slow wave circuit 5 is particularly advantageous because all of the end walls 8 are identical and merely assembled in different angular positions to define the composite coupled cavity slow wave circuit 5.

Upon an examination of the melting pattern produced by the individual beamlets, in the prior art tube of FIG. I, on the end wall of the last cavity as shown in FIG. 2, it is seen that the melting is produced in registration with the first one-third of the slot length as taken in the radial direction. Therefore, the missing slot web portion 22 need not cover the entire missing slot but may, in an alternative embodiment, adequately serve the purpose by blocking off the inner half of the slot 11. Thus, the missing slot portion of web 22 may comprise only a partial missing slot.

As one moves along the slow wave circuit 5 from the downstream end to the upstream end thereof, the electric fields of the wave decrease in intensity. Thus, the requirement for blocking off the individual arrays of coupling slots decreases. Therefore, the missing slot web portion 22 in the end walls 8 may be omitted near the upstream end of the slow wave circuit 5, such as in the upstream circuit portion 15.

Also, in certain hybrid circuit tubes, disclosed and claimed in U.S. Pat. No. 3,289,032 issued Nov. 29, 1966, not shown, the upstream portion 15 of the slow wave circuit 5 is replaced by a succession of klystron-type buncher cavities. In such a tube, the downstream circuit portion 16 serves as the output circuit. The missing slot web portion of the present invention is used to advantage to increase efficiency and to prevent arcing in such a hybrid tube.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a tube employing a coupled cavity slow wave circuit, means for projecting a beam of electrons over an elongated beam path, slow wave circuit means disposed along the beam path in electromagnetic energy exchanging relation with the beam, said slow wave circuit means including an array of cavity resonators arranged successively along the beam path with adjacent ones of said cavity resonators having common end wall structures, said common end wall structures having a plurality of generally radially directed elongated inductive coupling slots therein providing wave energy communication through said common end walls, said coupling slots being arranged in N angularly displaced axially aligned arrays around the beam path, each array being angularly displaced by 360/N where N is an integral number greater than 2, and wherein at least N number of end walls have less than N number of full-sized coupling slots therein defining a missing slot web portion of each of said end walls, said N number of end walls being arranged for blocking off at least a portion of a line-of-sight path parallel to the beam path through each of said axial arrays of coupling slots, whereby undesired beam paths through said coupling slots are blocked to inhibit cumulative electromagnetic interaction between undesired beamlets in said array of slots and fields of the slow wave circuit.

2. The apparatus of claim 1 wherein N is 8 and eight end walls have seven coupling slots therein and wherein the web portion of each of the common walls between the first and the seventh coupling slots in each end wall is angularly displaced by n 360IN relative to each of the other eight end walls, where n is an integral number.

3. The apparatus of claim 2 wherein each cavity resonator includes four conductive nose portions protruding radially into said cavity resonator from the sidewall thereof, said nose portions being angularly displaced about the beam axis from the adjacent noses by 90 to form a generally cloverleaf cavity geometric configuration, and said nose portions in adjacent cavity resonators being angularly displaced about the beam axis by 45 4. In a tube employing a coupled cavity slow wave circuit, means for projecting a beam of electrons over an elongated beam path, slow wave circuit means disposed along the beam path in electromagnetic energy exchanging relation with the beam, said slow wave circuit means including an array of cavity resonators arranged successively along the beam path with adjacent ones of said cavity resonators having common end wall structures, said common end wall structures having a plurality of generally radially directed elongated inductive coupling slots therein providing wave energy communication through said common and walls, said coupling slots being arranged in N angularly displaced axially aligned arrays around the beam path, each array being angularly displaced from the adjacent array by 360IN where N is an integral number greater than 2, and wherein a plurality of said end walls have less than N number of full-sized coupling slots therein defining a missing slot web portion of each of said end walls, said plurality of end walls being arranged such that said missing slot web portions blocking off at least a portion of a line-of-sight path parallel to the beam path through each of said axial arrays of coupling slots, whereby undesired beam paths through said coupling slots are blocked to inhibit cumulative electromagnetic interaction between undesired beamlets in said array of slots and fields of the slow wave circuit.

5. The apparatus of claim 4 wherein N is 8 and eight end walls have seven coupling slots therein and wherein the web portion of each of the common walls between the first and the seventh coupling slots in each end wall is angularly displaced by n 360/N relative to each of the other eight end walls, where n is an integral number.

6. The apparatus of claim 5 wherein each cavity resonator includes four conductive nose portions protruding radially into said cavity resonator from the side wall thereof, said nose portions being angularly displaced about the beam axis from the adjacent noses by to form a generally cloverleaf cavity geometric configuration, and said nose portions in adjacent cavity resonators being angularly displaced about the beam axis by 45. 

1. In a tube employing a coupled cavity slow wave circuit, means for projecting a beam of electrons over an elongated beam path, slow wave circuit means disposed along the beam path in electromagnetic energy exchanging relation with the beam, said slow wave circuit means including an array of cavity resonators arranged successively along the beam path with adjacent ones of said cavity resonators having common end wall structures, said common end wall structures having a plurality of generally radially directed elongated inductive coupling slots therein providing wave energy communication through said common end walls, said coupling slots being arranged in N angularly displaced axially aligned arrays around the beam path, each array being angularly displaced by 360*/N where N is an integral number greater than 2, and wherein at least N number of end walls have less than N number of full-sized coupling slots therein defining a missing slot web portion of each of said end walls, said N number of end walls being arranged for blocking off at least a portion of a line-of-sight path parallel to the beam path through each of said axial arrays of coupling slots, whereby undesired beam paths through said coupling slots are blocked to inhibit cumulative electromagnetic interaction between undesired beamlets in said array of slots and fields of the slow wave circuit.
 2. The apparatus of claim 1 wherein N is 8 and eight end walls have seven coupling slots therein and wherein the web portion of each of the common walls between the first and the seventh coupling slots in each end wall is angularly displaced by n 360* /N relative to each of the other eight end walls, where n is an integral number.
 3. The apparatus of claim 2 wherein each cavity resonator includes four conductive nose portions protruding radially into said cavity resonator from the sidewall thereof, said nose portions being angularly displaced about the beam axis from the adjacent noses by 90* to form a generally cloverleaf cavity geometric configuration, and said nose portions in adjacent cavity resonators being angularly displaced about the beam axis by 45* .
 4. In a tube employing a coupled cavity slow wave circuit, means for projecting a beam of electrons over an elongated beam path, slow wave circuit means disposed along the beam path in electromagnetic energy exchanging relation with the beam, said slow wave circuit means including an array of cavity resonators arraNged successively along the beam path with adjacent ones of said cavity resonators having common end wall structures, said common end wall structures having a plurality of generally radially directed elongated inductive coupling slots therein providing wave energy communication through said common end walls, said coupling slots being arranged in N angularly displaced axially aligned arrays around the beam path, each array being angularly displaced from the adjacent array by 360*/N where N is an integral number greater than 2, and wherein a plurality of said end walls have less than N number of full-sized coupling slots therein defining a missing slot web portion of each of said end walls, said plurality of end walls being arranged such that said missing slot web portions blocking off at least a portion of a line-of-sight path parallel to the beam path through each of said axial arrays of coupling slots, whereby undesired beam paths through said coupling slots are blocked to inhibit cumulative electromagnetic interaction between undesired beamlets in said array of slots and fields of the slow wave circuit.
 5. The apparatus of claim 4 wherein N is 8 and eight end walls have seven coupling slots therein and wherein the web portion of each of the common walls between the first and the seventh coupling slots in each end wall is angularly displaced by n 360*/N relative to each of the other eight end walls, where n is an integral number.
 6. The apparatus of claim 5 wherein each cavity resonator includes four conductive nose portions protruding radially into said cavity resonator from the side wall thereof, said nose portions being angularly displaced about the beam axis from the adjacent noses by 90* to form a generally cloverleaf cavity geometric configuration, and said nose portions in adjacent cavity resonators being angularly displaced about the beam axis by 45*. 